Photoaffinity labeling of biological macromolecules is widely used to identify specific receptor sites within complex units. This technique provides numerous advantages over more conventional labeling procedures. One of these is that photochemical activation of latent reactivity can generate labeling agents that are far more reactive than conventional ground-state chemical reagents. We will use nanosecond and picosecond laser spectroscopy to probe the details of the generation and subsequent transformations of the reactive molecular species responsible for covalent attatchment of the label to the biological macromolecule. We will attempt to define the structural features of the photoactivatable group that insure specific, irreversible labeling of the desired receptor. By studying the rates of reaction of the photogenerated species with specific functional groups typically found at biological receptor sites we will judge the utility and reliability of the reagent. Our investigation will focus on fundamental studies of aromatic azides, and of diazo compounds as these are the most common photochemicals used in labeling.